Need some advice on how to back out screws to allow for movement of table top

[Shift]

I'm about to start build a harvest table using 2" (1.5") pine ...

Lots of people suggest backing out the screws a bit when joining the table top to avoid cupping allow for seasonal movement.

My question is just how much do you back them out, and on which joints?

I'm assuming that it's only in the apron to table top joints, and not the edge joints between the actual table top pieces or any of the support structure. Do you back out the screws on the breadboards?

thanks

(corrected the title to avoid confusion for future readers)

[Shift]

Rick - is there anything that can be done to fill in those gaps?  There's a few on the top i'm building right now, and I can already see how those gaps and my 5yr old are going to get along all too well. Do people use wood filler to fill in gaps between planks? I'm using an gel resin stain, so, even if it did work, i'd need to find out if filler would take the stain or not.

Rick R said:

My first thought would be that the edges of the boards should be planed. Once they are flat, there should be no issue with cupping. I just built a tabletop using 2" thick cherry and had to dowel it and use pocket screws. I had problems wirh cupping until I planed the edges. I still had some gaps but overall it looks good and the wife is happy.

 

RR

[Jeff]

Hey, we got it sorted out, Shift--no worries.  As my grandfather used to say, "the day I don't learn something is the day I die".

Shift said:

Cupping was definitely the wrong word to use.

[Shift]

This article on table tops suggests using extra large pocket holes in the apron to allow for seasonal movement of the table top ... i'm guessing this was what the original advice i mentioned was referring to. 

Is there some Kreg equivalent I should be considering as I go to attach my table top?

[Rick]

Shift, there is some confusion here.  Seasonal movement should not take place between the boards that compose the table top -- they must be glued and screwed (or clamped) tightly together.

The movement the article is talking about is between the table top and the table frame.  The reason for allowing movement is simple -- because these boards are oriented perpendicular to each other. 

Humidity causes boards to swell perpendicular to their grain.  A 10 ft 2x4 will always be exactly 10 ft long no matter how humidity changes, but a wet 2x4 can easily grow from 3.5 in wide to 3.6 in wide.

So the supporting frame must allow the table top to grow and shrink -- across the grain -- with humidity, by allowing movements.

Shift said:

This article on table tops suggests using extra large pocket holes in the apron to allow for seasonal movement of the table top ... i'm guessing this was what the original advice i mentioned was referring to. 

[Pierre Vallee]

hm! how about torsional rigidity? some board will twist in the longitudinal direction. All depends on the grain and the cut… know I’m saying?

[Jeff]

To use the technique described in that article, you could just do the following:

1. Drill pockets in the apron with Kreg as usual.

2. Before screwing on the top, use a slightly larger drill to redrill the screw holes (NOT the pockets) in the apron.  In other words, you're just enlarging the part of the hole where the shank of the screw goes (the small end, deep in the pocket).  If you enlarge the pocket (the big end of the hole), you'll get no benefit other than a bigger place to put your screwdriver.

3. Screw the apron to the top.

(Whoever comes up with a way to add sketches to comments will make a fortune...)

Shift said:

This article on table tops suggests using extra large pocket holes in the apron to allow for seasonal movement of the table top ... i'm guessing this was what the original advice i mentioned was referring to. 

Is there some Kreg equivalent I should be considering as I go to attach my table top?

[Shift]

Thanks for the 101 Rick ... and  Jeff for breaking that method down in detail. I think we've now come full circle to my original post and can finally sort all of this out ...

I'm guessing that when I read the advice to back out the screws from the apron into the table top just slightly it was to give the screw a little more room to play for seasonal movement.

So here's the question ....

Would that actually work?

If not, how necessary is it to do what Jeff described? 

If I just attach the top to the apron using regular screw holes am I setting myself up for a lot of problems down the road?

I've seen a handful of examples of people using kreg pocket holes for attaching the apron to the top that don't make any allowances for movement in the top .... bad idea?

again thanks for all the insights and advice.

[Jeff]

To answer your question: no, backing off the screws slightly won't work, because it's the lateral shifting of the wood that causes the problem.  If the screws can't move laterally, there's no benefit.  Backing them off will reduce friction between the apron and top, but doesn't create allowance for lateral movement.  Picture the top getting bigger and smaller as the seasons change.  If the screws that are embedded in the top can't move in their holes, then the changing size of the top will distort either the top or the apron, whichever is weaker.  In the article you referenced, the method of using little cleats inserted into slots in the apron most clearly illustrates how the top isn't rigidly attached to the apron.  That's the goal: attachment, but not rigid attachment.

To be completely honest, I've not had any issues with distortion of tops, even with quick ones I've just knocked together on the apron with regular Kreg joints.  I suspect that modern polyurethane finishes go a long way toward preventing humidity-related distortion, simply because they prevent absorption of water and therefore expansion of the wood.  Finish the top before you do final assembly, and it's basically wrapped in a nice watertight covering of poly.  That, along with central air conditioning, means that furniture doesn't get subjected to anywhere near the kind of seasonal environmental shifts that used to be commonplace. 

If you're not using a poly finish and don't have central air, please ignore this post.  ;) 

[Shift]

Big thanks Jeff.

Funny thing is I'm not using a poly finish and only use central air occasionally :)

I'm using bioshield's aqua resin gel stain ... not sure how much protection that's going to give but i don't think it will be nearly as much as a poly finish.

[Rick]

The only time I saw a piece literally self-destruct was when my friend glued up a "breadboard" to the ends of a small table top, not realizing that wood needs to move.  (See page 36 in the link http://www.finewoodworking.com/media/TabletopsFlat.pdf )

Note that the FWW article specifically instructs to glue the breadboard only in the center 4 inch area.  Obviously my friend glued up the entire length, and as the boards shrunk in width, cracks developed.

Jeff said:

To be completely honest, I've not had any issues with distortion of tops

[Shift]

Interesting ... I attached my breadboards with screws/pocket holes all the way across ....... sounds like I could be in for some serious trouble.

Rick said:

The only time I saw a piece literally self-destruct was when  .....

[Rick]

Shift, going back to your original post, I assumed that you were using pocket screws to connect parallel boards together.  These would never be loosened for movement.  Neither would pocket screws that fasten plywood or MDF to boards along the grain.  The problem, as we have discussed, is the need for cross-grain movement in real wood. 

Did you glue the breadboards to the ends of the table tops??

Shift said:

Interesting ... I attached my breadboards with screws/pocket holes all the way across ....... sounds like I could be in for some serious trouble.